1. Field of the Invention
The present invention relates to an apparatus and method for detecting reception signal symbol synchronization in a wireless communication system; and, more particularly, to an apparatus and method which prevents deterioration of synchronization performance due to difference of time responses occurring in each reception antenna and variation of delay times occurring in each frequency band when a symbol of a very high-speed wireless communication system using a multiple antenna or a multiple frequency band is synchronized.
This work was supported by the IT R&D program for MIC/IITA [2006-S-002-02, “IMT-Advanced Radio Transmission Technology with Low Mobility”].
2. Description of Related Art
Research for overcoming limitation of IMT-2000 and building a fourth generation mobile communication network has been actively progressed. The fourth generation mobile communication network is not a single communication network but a compounding communication network and has a format that diverse communication networks such as satellite communication, wireless Local Area Network (LAN), and digital broadcasting are integrated.
International Telecommunication Union (ITU) regulates that the fourth generation wireless communication provides a transmission speed of 100 Mbps while moving and a transmission speed of 1 Gbps while staying.
There are Institute of Electrical and Electronics Engineers (IEEE) 802.11b, IEEE 802.11a, IEEE 802.11n, and IEEE 802.11g as a current wireless LAN. The currently used wireless LAN satisfies IEEE 802.11b or IEEE 802.11g standard. The IEEE 802.11g supports a transmission speed ranging from 2.4 GHz band to maximum 54 Mbps. IEEE 802.11n, which is currently being developed, is expected to support a transmission speed of 540 Mbps maximally which does not satisfy the transmission speed of 1 Gbps required by the fourth generation wireless communication. Orthogonal Frequency Division Multiplexing (OFDM) and Multiple-Input Multiple-Output (MIMO) antenna technologies are core technologies for realizing the very high-speed wireless communication.
NTT DoCoMo, Inc. expects that Orthogonal Frequency Code Division Multiplexing (OFCDM) and OFDM may be used as technology for the fourth generation wireless communication. In a communication system using a broadband channel, OFCDM and OFDM show superior performance than wireless communication access based on typical Direct Sequence-Code Division Multiple Access (DS-CDMA). Performance deterioration due to interference by multi-path is remarkably reduced in OFCDM or OFDM.
Spectral efficiency should be improved to raise a transmission speed in a state with limited frequency resources. Multiple antenna technology is used as one method for improving the spectral efficiency. A transmission method such as Bell Laboratories Layered Space Time (BLAST) is suggested in order to use the multiple antenna technology. It is already known that Maximum Likelihood Detection (MLD) has better performance than V-BLAST or Minimum Mean Squared Error (MMSE) in a bit error rate or a block error rate. However, since computational complexity is very high due to exponential increase according to a modulation method and the number of antennas, it is not proper to be realized in an actual communication system. QRM-MLD, which is MLD using M-algorithm and QR decomposition, is suggested as a method for reducing complexity, which is the problem of the MLD. However, there is a possibility to be improved more.
A New Nomadic Local Area Wireless Access (NoLA) system developed in Electronics and Telecommunications Research Institute (ETRI) satisfies the transmission speed required in the fourth generation wireless communication by using the multiple antenna technology and OFDM. The NoLA system secures a transmission speed of 3.6 Gbps by applying the multiple antenna technology using 4 or 8 transmission antennas and 8 reception antennas, and using 3 frequency bands of 40 MHz in a carrier wave frequency of 5 GHz., i.e., by using a frequency band of total 120 MHz.
The NoLA system has four characteristics.
First, there is multiple antenna technology. The multiple antenna technology is a method for acquiring high spectral efficiency through antenna diversity and is used in a system for high-speed wireless communication such as IEEE 802.11g and IEEE 802.11n. IEEE 802.11n uses 4 transmission and reception antennas but the NoLA system uses 8 transmission and reception antennas to provide a higher transmission speed. The transmission speed is raised by using a plurality of antennas but it causes a problem that complexity of a receiving end such as a detector highly increases.
A second characteristic of the NoLA system is to raise a reception rate as a method for acquiring data of high reliability in a few antennas by using Maximum Density Droplet (MDD) developed by a next generation wireless LAN team of ETRI and removing interference of data received in the antennas through Successive Interference Cancellation (SIC).
A third characteristic of the NoLA system is to use Low Density Parity Check (LDPC) code as a channel coding method. Since the LDPC code is capable of performing a parallel process, it is proper to process 3 Gbps data at high-speed.
A fourth characteristic of the NoLA system is to use an OFDM method which is being actively researched as a method proper to high-speed data transmission in a wired/wireless channel. Since the OFDM method uses a plurality of carrier waves having mutual orthogonality, spectral efficiency can be raised. Since a procedure for modulating/demodulating the carrier waves in transmitting/receiving ends is the same as performing Inverse Discrete Fourier Transform (IDFT) and DFT, the OFDM method can be realized at high-speed based on Inverse Fast Fourier Transform (IFFT) and FFT. Since the OFDM method is proper to high-speed data transmission, it is adopted as a standard method for the high-speed wireless LAN of IEEE 802.11a and HIPELAN/2, broadband wireless access of IEEE 802.16, digital audio broadcasting, digital terrestrial television broadcasting, Asymmetric Digital Subscriber Line (ADSL), and Very High Bitrate DSL (VDSL).
As described in the very high-speed wireless communication technologies and the NoLA system above, it is necessary to use the multiple antenna technology and the multiple frequency band to perform very high-speed wireless communication faster than the transmission speed required in the fourth generation wireless communication. At this time, an efficient symbol synchronization method appropriate for the wireless communication system using the multiple antenna and the multiple frequency band is required.
It is well known that a method for capturing a synchronization timing by detecting a symbol boundary as a threshold detection technique using auto-correlation, which is one of the symbol synchronization methods, is a very efficient technique when one antenna and one frequency band are used. However, when the multiple antenna or the multiple frequency band are used, reception antennas have different time responses of the auto-correlation function. Also, when hardware processing the frequency band is configured as an individual board and a plurality of boards are used, there is a problem that the frequency bands have different delay times in signal response.
Variation between the time response and the delay time causes deterioration of synchronization performance as a factor that contributes to the infringement of a guard interval realized as Cyclic Prefix (CP) in the OFDM-based very high-speed wireless communication system of the Gbps level.